The present invention relates to a novel process for preparing, by electrochemical reduction, a carbon-containing material whose surface is modified with organic groups, in particular functionalized organic groups, this process comprising placing the carbon-containing material in contact with organic diazonium salt in solvent, optionally in the presence of an electrolyte, and negative polarization of the carbon-containing material relative to an anode which is also in contact with a solution of the said organic diazonium salt or in contact with an electrolytic solution separated by a suitable separator for the solution of the said salt.
The invention also relates to the modified carbon-containing materials obtained by the said process and to the use of these carbon-containing materials, in particular for the preparation of composite materials or for carrying out chemical or biological reactions, or for the complexation of metals.
European patent EP-B-569,503 has already described a process for preparing a carbon-containing material whose surface is modified with aromatic groups, in particular functionalized aromatic groups, by electrochemical reduction.
This process is characterized in that it consists in binding an aromatic group to the surface of this material by electrochemical reduction of a diazonium salt comprising this aromatic group, by placing the carbon-containing material in contact with a solution of the diazonium salt in an aprotic solvent and negatively polarizing the carbon-containing material relative to an anode which is also in contact with the diazonium salt solution.
This document furthermore indicates that, in order for the electrochemical reduction of the diazonium salt to lead to binding of the aromatic group of this salt to the surface of the carbon-containing material, it is necessary to carry out the reduction in an aprotic medium which contains neither any nucleophilic compounds nor any ethylenic compounds nor any species capable of reacting with the neutral radical faster than this radical binds to the carbon-containing surface, and to perform the process at a potential which is more negative than the reduction potential of the diazonium salt.
It has now been found, unexpectedly, that the use of this process in protic solvent in acidic medium leads to the binding of an organic group to the surface of the carbon-containing material.
The process according to the invention is thus characterized in that the electrochemical reduction is carried out on an organic diazonium salt in protic solvent in acidic medium.
The electrochemical reduction can be represented schematically according to the following reaction:
RN2++xcex8xe2x80xa2xe2x86x92RN2xe2x80xa2xe2x86x92Rxe2x80xa2+N2
R being an organic residue.
The residue R binds to a carbon atom at the surface of the carbon-containing material. The covalent bond formed is of the type: carbon of the carbon-containing material-carbon of the organic residue.
According to the process of the invention, the cathode potential is set at a value such that the neutral radical Rxe2x80xa2 is not reduced and does not give a carbanion. The potential should thus be adjusted to a value which is not too negative, in order to stabilize the neutral radical Rxe2x80xa2.
It will be noted that the term xe2x80x9cresiduexe2x80x9d denotes the organic part of the diazonium.
It is understood that the invention covers all diazonium salts whose residues R in radical form will be stable enough to bind to a carbon of the carbon-containing material.
The term xe2x80x9cgroup Rxe2x80x9d will denote the residue R bound to the carbon-containing material, it being understood that the group R will have a broader meaning than the residue R due to the subsequent chemical conversions which it may undergo, as will be understood in the light of the description which follows.
The invention relates in particular to a process characterized in that the diazonium salt corresponds to the formula:
ArN2+Xxe2x88x92xe2x80x83xe2x80x83(I)
in which:
Ar is an optionally substituted C6-C14 aromatic residue or an optionally substituted heteroaromatic residue of 5 to 14 atoms, comprising one or more hetero atoms chosen from oxygen, nitrogen, sulphur and phosphorus,
Xxe2x88x92 is an anion chosen from halogens, sulphates, phosphates, perchlorates, tetrafluoroborates, carboxylates and hexafluorophosphates.
The term xe2x80x9caromatic residuexe2x80x9d means any residue comprising one or more C6-C14 fused or independent benzenic nuclei known per se. Non-limiting examples which may be mentioned are phenyl, naphthyl and anthryl nuclei, triple-fused nuclei, biphenyl nuclei, etc.
The term xe2x80x9cheteroaromatic residuexe2x80x9d means any aromatic heterocycle comprising one or more hetero atoms such as N, O, S or P, in particular comprising 5 to 14 atoms.
In fact, it is understood that the invention is not limited to specific compounds, but, on the contrary, extends to any diazonium salt which can be reduced to give a neutral radical Arxe2x80xa2.
Among the aromatic residues which may be mentioned, by way of example, are aromatic residues whose substituents are chosen from the group consisting of:
linear or branched aliphatic radicals optionally comprising one or more double bond(s), optionally substituted with carboxyl, NO2, disubstituted protected amino, monosubstituted protected amino, cyano, diazonium, alkoxy, alkoxycarbonyl, alkylcarbonyloxy or optionally fluorinated vinyl radicals or halogen atoms,
aryl radicals optionally substituted with carboxyl, NO2, disubstituted protected amino, monosubstituted protected amino, cyano, diazonium, alkoxy, alkoxycarbonyl, alkylcarbonyloxy or optionally fluorinated vinyl radicals or halogen atoms,
carboxyl, NO2, disubstituted protected amino, monosubstituted protected amino, cyano, diazonium, alkoxy, alkoxycarbonyl, alkylcarbonyloxy or optionally fluorinated vinyl radicals or halogen atoms.
As regards the optionally fluorinated vinyl radicals, these should be compatible with the envisaged reaction of the diazonium.
The term xe2x80x9cfunctionalizedxe2x80x9d means in particular that the aromatic or heteroaromatic or aliphatic radicals, when the aromatic or heteroaromatic radicals are substituted with an aliphatic radical, comprise one or more substituent(s) capable of reacting with a substrate or one or more substituent(s) capable of being converted into substituents capable of reacting with a substrate.
These substituents can thus be very varied depending on the applications for which the carbon-containing materials are intended.
Among the substituents capable of reacting directly with a resin, in particular an organic resin, mention may be made, for example, of xe2x80x94(CH2)nxe2x80x94COOH, xe2x80x94(CH2)nxe2x80x94CH2xe2x80x94OH and (CH2)nxe2x80x94NH2 groups, n being an integer between 0 and 10, and xe2x80x94CHxe2x95x90CH2 and xe2x80x94CFxe2x95x90CF2 groups.
Among the precursor substituents which, after conversion, are capable of reacting with a resin, in particular an organic resin, mention may be made, for example, of NO2, N2+, (CH2)nxe2x80x94CN, (CH2)nxe2x80x94CHO and (CH2)nxe2x80x94COOPr groups, Pr being a protecting group, and (CH2)nxe2x80x94NHPxe2x80x2r, (CH2)nxe2x80x94N(Pxe2x80x2r)2 and (CH2)nxe2x80x94Nxe2x95x90Pxe2x80x3r groups, Pxe2x80x2r and Pxe2x80x3r being protecting groups and n being an integer between 0 and 10.
Among the substituents capable of reacting directly with a biological molecule, mention may be made of xe2x80x94(CH2)nxe2x80x94COOH and xe2x80x94(CH2)nxe2x80x94NH2 groups, n being an integer between 0 and 10.
Among the precursor substituents which, after conversion, are capable of reacting with a biological molecule, mention may be made of NO2, NO2, N2+, (CH2)nxe2x80x94CN, (CH2)nxe2x80x94CHO and (CH2)nxe2x80x94COPr groups, Pr being a protecting group and n being an integer between 0 and 10.
Among the substituents capable of reacting directly with functional organic molecules, mention may be made of NO2, (CH2)nxe2x80x94CONH2, (CH2)nxe2x80x94CN, (CH2)nxe2x80x94CHO, (CH2)nxe2x80x94COOH, (CH2)nxe2x80x94CH2OH and (CH2)nxe2x80x94NH2 groups, n being an integer between 0 and 10, and SO2H, SO3H, SO2R and SO3R groups, R being an aliphatic or aromatic carbon-containing chain of 1 to 20 carbon atoms.
Among the precursor substituents which, after conversion, are capable of reacting with functional organic molecules, mention may be made of N02, (CH2)nxe2x80x94CONH2 and (CH2)nxe2x80x94COOPr groups, Pr being a protecting group, and (CH2)nxe2x80x94NHPxe2x80x2r, (CH2)nxe2x80x94N(Pxe2x80x2r)2 and (CH2)nNxe2x95x90Pxe2x80x3r groups, Pxe2x80x2r and Pxe2x80x3r being protecting groups, and (CH2)nxe2x80x94CN, (CH2)nxe2x80x94CHO, (CH2)nxe2x80x94COOH and (CH2)nxe2x80x94CH2OH groups, n being an integer between 0 and 10, and SO2Pr and SO3Pr groups, Pr being a protecting group chosen from the meanings of R.
Besides the functional substituents, the aromatic or heteroaromatic residues can comprise one or more unreactive substituents such as alkyl or alkenyl radicals.
The term xe2x80x9calkyl radicalxe2x80x9d or xe2x80x9calkenyl radicalxe2x80x9d means, in particular, linear or branched radicals comprising 1 to 20 carbon atoms, optionally one or more hetero atom(s), and optionally one or more double bond(s). These radicals can also comprise one or more halogen atom(s). By extension, the radicals also comprise cycloaliphatic radicals.
By way of example, mention is made of the following diazonium salts:
Compound 1: 4-nitrobenzenediazonium tetrafluoroborate
Compound 2: 3,3xe2x80x2-dimethoxybiphenyl-4,4xe2x80x2-bis(diazonium) dichloride of formula: 
(Fast Blue B crystallized with a molecule of ZnCl2)
Compound 3: 4-carboxymethylbenzenediazonium tetrafluoroborate Compound 4: 1,4-benzenebis(diazonium) tetrafluoroborate
Compound 5: chlorobenzyl-4-diazonium tetrafluoroborate.
Other examples of diazonium salts which can be used in the present invention are chosen from 4-chloromethylphenyldiazonium, 4-hydroxymethylphenyldiazonium, 4-carboxyphenyldiazonium, 4-formylphenyldiazonium, 4-acetylphenyldiazonium, 4-isothiocyanatophenyldiazonium, 4-N-FMOC-aminomethylphenyldiazonium, 4-(4-hydroxymethylphenoxymethyl)phenyldiazonium, 4-(2,4-dimethoxyphenyl-N-FMOC-aminomethyl)phenyldiazonium, 4-(phenyl-N-FMOC-aminomethyl)phenyldiazonium, 4-(4-methylphenyl-N-FMOC-aminomethyl)phenyldiazonium and 4-(4-nitrophenylcarbonyl)phenyldiazonium salts, trityldiazonium chloride, 2-chlorotrityldiazonium chloride, trityldiazonium hydroxide, 9-N-FMOC-aminoxanthen-3-yldiazonium, 4-(2,4-dimethoxyphenylhydroxymethyl)phenyldiazonium, 4-(4-hydroxymethylbenzoyloxymethyl)phenyldiazonium, 4-(4-hydroxymethylbenzoylaminomethyl)phenyldiazonium, 4-(4-hydroxymethyl-3-methoxyphenoxymethyl)phenyldiazonium, etc. salts.
The negative electrode consisting of the carbon-containing material should be at a cathode potential value such that it can donate an electron to the diazonium.
This value can be measured relative to a reference electrode and it is thus understood that this potential value will vary depending on the diazonium salt.
In practice, according to one advantageous process, the cathodic reduction of the diazonium salts is carried out by repetitive cyclic voltammetry in a potential range in which they are reduced or by electrolysis at a potential which is more negative than the reduction potential of the diazonium salt.
Among the protic solvents which may be mentioned, for example, are water, methanol, ethanol or mixtures thereof or mixtures with aprotic solvents, for example acetonitrile, it being understood that this resulting mixture has the characteristics of an aprotic solvent.
According to one preferred embodiment, the solution can optionally comprise an electrolyte such as quaternary ammonium salts or alkali metal salts which are soluble in the medium.
Among these salts, mention may be made of quaternary ammonium or alkaline halides, acetates, tetrafluoroborates, perchlorates and hexafluorophosphates, in particular lithium tetrafluoroborate or a (C1-C4) alkylammonium tetrafluoroborate such as tetraethylammonium tetrafluoroborate.
The electrolyte can be simply an acid chosen from hydrochloric acid, sulphuric acid, nitric acid, nitrous acid, phosphoric acid and tetrafluoroboric acid.
The diazonium salt concentration and electrolyte concentration are generally between 10xe2x88x923 mol/l and 10xe2x88x921 mol/l for the diazonium salt and between 10xe2x88x922 mol/l and 1 mol/l for the electrolyte.
The acids can be chosen from inorganic acids such as hydrochloric acid, sulphuric acid, nitric acid, nitrous acid, phosphoric acid or tetrafluoroboric acid, or organic acids. The pH is advantageously less than 2.
The modified carbon-containing materials are recovered and can be characterized in several ways.
Mention is made of cyclic voltammetry, X-ray photoelectron spectroscopy and tunnelling microscopy.
The measurements taken confirm the binding of the Ar groups to the carbon-containing surface. The surface density of the Ar groups depends on the steric bulk of these groups and will thus vary depending on the Ar groups considered.
By way of example, unexpectedly, it has been found that the electrochemical reduction, under the conditions of the invention, of an aromatic diazonium salt comprising one or more nitro substituents leads directly to binding of the aromatic group and conversion of the nitro group(s) into NH2 group(s) according to the following reaction: 
By way of example, 4-nitrobenzenediazonium salts are mentioned.
According to another variant, the carbon-containing conductive materials whose surface has been modified by the process according to the invention are subjected to a subsequent conversion of the functional substituents. For example, the substituents may be converted in order to react with a resin, a biological molecule or a functional organic molecule.
The invention thus covers carbon-containing materials converted after the process according to the invention.
The invention also relates to the carbon-containing materials which can be obtained by the process according to the invention.
According to one variant, these modified carbon-containing materials, in particular made of graphite or of glass carbon, are in the form of fibers, powder, felt, fabric, beads or carbon/carbon composite. When the materials are in the form of fibers, one advantageous application consists in producing improved composite materials of fibers/resin such as an epoxy resin.
The invention also relates to carbon-containing materials modified with groups R in which the substituents are capable of covalently binding metal cations, chemical compounds such as complexing agents or biological molecules, such as proteins and in particular enzymes.
According to one advantageous application, the materials at the surface of which are bound functional organic groups capable of reacting with a biological molecule are used for carrying out biological reactions.
According to another advantageous application, the materials at the surface of which are bound functional organic groups capable of reacting with a complexing or functional organic molecule are used for carrying out such reactions.
One more particularly advantageous application is especially the application of the process according to the invention to combinatorial chemistry. Combinatorial chemistry can be defined as the preparation of a set of molecules known as a xe2x80x9clibraryxe2x80x9d, it being possible for the number of which to range from a few compounds to several thousands. Combinatorial synthesis constitutes a new strategy for systematically and simultaneously generating a very large number of molecules and studying the structure-activity relationship [F. Balkenhohl et al., Combinatorial Stnthesis of Small Organic Molecules, Angew. Chem. Int. Ed. Engl., 1996, 35, pp. 2288 -2337].
Combinatorial chemistry is usually carried out on a solid support [J. S. Frxc3xcchtel and G. Jung, Organic Chemistry on Solid Supports, Angew. Chem. Int. Ed. Engl., 1996, 35, pp. 17-42]. To date, the supports most commonly used are polystyrene supports, combined with 1-2% of divinylbenzene or of polystyrene/polyethylene glycol (PEG-PS) copolymer [L. A. Thompson and J. A. Ellman, Synthesis and Application of Small Molecule Libraries, Chem. Rev., 1996, 96, pp. 555-600]. Thus, another subject of the present invention is the application of the carbon-containing materials obtained according to the electrochemical process as defined above, in combinatorial chemistry, and more particularly the application of the carbon-containing materials at the surface of which are bound organic groups capable of reacting with functional organic molecules to make a library of organic compounds. Such a use is particularly advantageous in the sense that it constitutes an alternative to conventional polymer supports.
In practice, such a use consists in grafting functionalized organic groups as defined above onto a carbon-containing material according to the electrochemical reduction process of the present invention, and then in reacting the said organic groups with various functional organic molecules in order to make a combinatorial chemistry library, and finally in cleaving the compounds obtained from the surface of the carbon-containing material.
The reactions between the organic groups grafted onto the carbon-containing material and the functional organic molecules are all compatible reactions which are known to those skilled in the art.
The carbon-containing materials which can be used more particularly in combinatorial chemistry are those which have an optimum working surface. For example, mention may be made of carbon felts.
In this specific case of the use of the process according to the present invention in combinatorial chemistry, it is understood that it is not limited to the grafting of specific compounds, provided that it concerns an organic diazonium salt in solvent. Many compounds can thus be applied to combinatorial chemistry on a carbon-containing support. Mention may be made, for example, of the following salts: 4-chloromethylphenyldiazonium, 4-hydroxymethylphenyldiazonium, 4-carboxyphenyldiazonium, 4-formylphenyldiazonium, 4-acetylphenyldiazonium, 4-isothiocyanatophenyldiazonium, 4-N-FMOC-aminomethylphenyldiazonium, 4-(4-hydroxymethylphenoxymethyl)phenyldiazonium, 4-(2,4-dimethoxyphenyl-N-FMOC-aminomethyl)phenyldiazonium, 4-(phenyl-N-FMOC-aminomethyl)phenyldiazonium, 4-(4-methylphenyl-N-FMOC-aminomethyl)phenyldiazonium and 4-(4-nitrophenylcarbonyl)phenyldiazonium salts, trityldiazonium chloride, 2-chlorotrityldiazonium chloride, trityldiazonium hydroxide, 9-N-FMOC-aminoxanthen-3-yldiazonium, 4-(2,4-dimethoxyphenylhydroxymethyl)phenyldiazonium, 4-(4-hydroxymethylbenzoyloxymethyl)phenyldiazonium, 4-(4-hydroxymethylbenzoylaminomethyl)phenyldiazonium, 4-(4-hydroxymethyl-3-methoxyphenoxymethyl)phenyldiazonium, etc. salts.
Other molecules which can be used are also cited in P. H. H. Hermkens et al., Solid-Phase Reactions: A Review of the Recent Literature, Tetrahedron, 1996, Vol. 52, pp. 4527-4554, and P. H. H. Hermkens et al., Solid-Phase Reactions II : A Review of the Literature November 95-November 96, Tetrahedron, 1997, Vol. 53, pp. 5643-5678.
Among the modified carbon-containing materials which may be mentioned, for example, are those in which the group R corresponds to the formula Ar, Ar being an optionally substituted C6-C14 aromatic group or a heteroaromatic group of 5 to 14 optionally substituted atoms comprising one or more hetero atoms chosen from oxygen, nitrogen, sulphur and phosphorus.
The detailed description of these groups is given in the process section of the account of the invention.
According to one variant, the groups Ar are substituted with SH, SO2H, SO3H, SO2R or SO3R radicals.
The invention also relates to modified carbon-containing materials made in particular of carbon black, of highly oriented pyrolytic graphite (HOPG) or of glass carbon, which are, according to one variant, in the form of fibers, powders, felt, fabric, beads or carbon/carbon composites.
Preferably, the fibers comprise substituents capable of reacting covalently with radicals present on an organic resin intended to be combined with the said fibers in the form of composite materials.
Needless to say, the nature of these substituents is generally very varied and depends on the organic resin considered.
The invention also relates to the composite materials comprising an organic resin matrix reinforced with modified carbon fibers according to the invention.
The matrix of a composite material can be a thermosetting polymer such as an epoxy resin, or a thermoplastic resin, for example a polyamide, polyethylene or polytetrafluoroethylene resin.
Among the substituents capable of reacting directly with a biological molecule, mention may be made, by way of example, of COOH and NH2 groups.
The process according to the invention can be carried out using a known electrolysis cell, such as the one described in patent EP-B-569,503, FIG. 1 and description page 8, lines 12 to 25, the content of which is incorporated by reference, or using a cell comprising separate compartments.
The examples below illustrate the invention.
All the examples are carried out in an electrolysis cell into which is introduced an electrolyte solution and one of the following diazonium salts:
Compound 1: 4-nitrobenzenediazonium tetrafluoroborate
Compound 2: 3,3xe2x80x2-dimethoxybiphenyl-4,4xe2x80x2-bis(diazonium) dichloride of formula: 
(Fast Blue B crystallized with a molecule of ZnCl2)
Compound 3: 4-carboxymethylbenzenediazonium bis(tetrafluoroborate)
Compound 4: 1,4-benzenebis(diazonium) bis(tetrafluoroborate)
Compound 5: chlorobenzyl-4-diazonium tetrafluoroborate.
The acidic aqueous medium used as solvent is water in the presence of hydrochloric acid, sulphuric acid or nitric acid.
In most cases, the acid serves as electrolyte.
The experiments are carried out on glass carbon (GC) electrodes.